In a liquid crystal display devices which has a color display panel, pixels are arranged in a row direction (X-direction) and a column direction (Y-direction). The X-direction intersects with the Y-direction. Each pixel (which may be referred to as a pixel circuit) comprises a color filter and functions as a red (R) subpixel, a green (G) subpixel or a blue (B) subpixel.
Display panels comprising a memory in each subpixel have been developed.
In each liquid crystal display device, for example, gate lines parallel to the X-direction are arranged at regular intervals in the Y-direction. Source lines parallel to the Y-direction are arranged at regular intervals in the X-direction. The pixels are provided near the respective intersections between the gate lines and the source lines.
Some liquid crystal display devices comprise a digital memory in each subpixel. This type of liquid crystal display device does not need to frequently supply voltage to all the source lines (in other words, does not need to rewrite pixel signals) when a static image is displayed in the entire display area of the device for a long time. Thus, power consumption can be reduced in the device. In addition, when a static image is displayed in a part of the display area, and moving images are displayed in the remaining part, there is no need to frequently apply voltage to all the source lines. In this case, voltages (in other words, pixel signals for moving images) need to be applied to the source lines of the partial area in which the moving images are displayed. In this manner, power consumption can be reduced in the device.
In some cases, the whole screen of the display device should be switched to, for example, white display or black display at once. If the normal image display has been switched to black or white display on the whole screen, the eye-catching effect or highlighting effect can be enhanced in the display device. For example, when the above display switching is performed by a display device which shows the information of an item of merchandise on a display shelf at a store, it is possible to draw the attention of customers to the commodity.
To perform the display switching, the memory data of subpixels of the entire display area need to be rewritten to data for black or white. In a case of active-matrix driving, a line is specified perpendicularly in series. In this manner, data for black or white is written to each line. Thus, time is required to complete rewriting the whole screen. In addition, power is consumed during this period. If the frequency of rewriting is slow, images in the process of rewriting are visually recognized as an image lag.